Defense and commercial electronic systems such as radar surveillance, terrestrial and satellite communications, navigation, identification, and electronic counter measures are often deployed on a single structure such as a ship, aircraft, satellite or building. These systems usually operate at different frequency bands in the electromagnetic spectrum. To support multiple band, multiple function operations, several single discrete antennas are usually installed on separate antenna platforms, which often compete for space on the structure that carries them. Additional antenna platforms add extra weight, occupy volume, and can cause electromagnetic compatibility, radar cross section, and observation problems.
There is a need to operate antenna apertures at close proximity to each other at different frequencies and with different functions, without detrimentally affecting antenna operation. It is often desired to have multiple band, wide scan, and multiple channel capabilities in a single platform. A typical architecture for providing multiple band, multiple function capabilities in a single platform is shown in FIG. 1. The antenna platform 100 comprises multiple antenna cells 110A . . . N, where each cell consists of a radiating element 116A . . . N, a transmission line 114A . . . N that couples RF energy to the radiating element 116A . . . N, and a radiating control element 112A . . . N, such as a phase shifter, transmit and receive (T/R) module, or other devices that control the RF energy radiated from each radiating element 116A . . . N. Each antenna cell 110A . . . N is coupled to a separate transmit or receive function 10A . . . N. Each transmit or receive function 10A . . . N an independent process of amplitude, phase, and/or frequency. For example, one function may the transmission of a satellite communication signal at 2 GHz, while another function may be the receipt of a radar signal at 10 GHz. The antenna platform 100 may comprise a planar array that contains several of the antenna cells 110A . . . N latticed in two dimensions, with each cell 110A . . . N acting collectively to produce a far field beam related to the overall desired functional properties.
An antenna platform may use a different density of antenna cells occupying the same lattice space for different transmit or receive functions. For example, a high frequency function, such as a radar operating at 10 GHz, may use several antenna cells to provide for precision beam steering, while a low frequency function, such as a communication channel operating at 2 GHz, may use fewer antenna cells due to its lower wavelength. The use of different densities of antenna cells for different functions is sometimes referred to as array thinning. Each transmit or receive function may require a unique lattice spacing to optimize radiation performance, such as to provide grating lobe free scanning, or to optimize beam width synthesis. At lower frequencies, phase control over fewer radiating elements is required to achieve grating lobe free scanning, since only elements spaced more than a half wavelength apart must be controlled.
FIG. 2 illustrates a planar array 200 where different densities of antenna cells 210A, 210B, 210C are used for three different antenna functions, 10A, 10B, 10C. In FIG. 2, a specific area of the planar array 200, a first function 10A uses four antenna cells 210A, while a second function 10B uses only two radiating elements 210B, while a third function 10C uses only a single antenna cell 210C. Each antenna cell 210A, 210B, 210C still contains a radiating element 216A, 216B, 216C, a transmission line 214A, 214B, 214C, and a radiating control element 212A, 212B, 212C.
Note that thinning the array reduces the number of elements required in the planar array. For example, if a planar array uses sixteen antenna cells for each function, and the array services three functions, a total of forty-eight antenna cells are required for the array. This also means that forty-eight radiating elements, transmission lines, and radiating control elements are also required. However, if the array thinning illustrated in FIG. 2 is used, fewer antenna cells and thus fewer antenna components are required. For example, in FIG. 2, if the first function 10A uses a total of sixteen antenna cells 210A to achieve the desired performance, sixteen radiating elements 216A, transmission lines 214A, and radiating control elements 212A are required. However, the second function 10B will require only half as many antenna cells 210B, so it requires only eight radiating elements 216B, transmission lines 214B, and radiating control elements 212B. Finally, the third function 10C requires one-quarter as many antenna cells 210C as the first function 10A, so it requires only four radiating elements 216C, transmission lines 214C, and radiating control elements 212C. Hence, the array thinning shown in FIG. 2 provides a significant reduction in the number of components.
Antenna cells of a thinned planar array can be interleaved in a single array as shown in FIG. 2. However, if the radiating elements are in close proximity to each other, the RF energy from an antenna cell supporting one function is likely to couple to another antenna cell and reduce the performance of the array. One approach to reduce the coupling of RF energy is to switch the unused cells, as shown in FIG. 3. In FIG. 3, each antenna cell 310A,B,C in the planar array 300 consists of a radiating control element 312A,B,C, an RF switch 318A,B,C, a transmission line 314A,B,C, and a radiating element 316A,B,C. However, simply disconnecting an unused cell 310A,B,C with the RF switch 318A,B,C, is not desired because the finite length of open circuit transmission lines 314A,B,C tends to add spurious impedance to the array 300, or losses can occur when the switches 318A,B,C are terminated in loads.
The prior art discloses many techniques for addressing the interleaving problems discussed above without the use of switches. Provencher et al. in U.S. Pat. No. 3,623,111, Bowen et al. in U.S. Pat. No. 4,772,890, Chu et al. in U.S. Pat. No. 5,557,291, and Mott et al. in U.S. Pat. No. 5,461,392 disclose examples of multiple band arrays that do not use switches to provide operation at multiple frequency bands. These arrays generally use radiating elements configured to radiate radio frequency energy at a specific frequency band. Dissipation of the active ports is minimized by reducing the coupling of energy into adjacent inactive radiating elements. Because the adjacent elements in an interleaved aperture can re-radiate spurious signals with an amplitude and phase varying over frequency, thus interfering with the radiation of the desired signal, the apertures within these arrays are usually cross-polarized from one another or widely spaced in frequency to avoid mutual coupling errors. However, these design choices limit the flexibility of the array.
The prior art also discloses reusing radiating elements at lower frequency bands by coupling the radiating elements with the transmit or receive function with an RF combiner 460, such as a coupler, diplexer, or switch, as shown in FIG. 4. FIG. 4 shows an antenna array 400 where three transmit or receive functions 10A,B,C are coupled to separate radiating control elements 420A,B,C. However, the outputs of the radiating control elements 420A,B,C are multiplexed to the minimum number of radiating elements 440 required to support a specific function 10A,B,C by using RF combiners 460. In the example depicted in FIG. 4, one function 10A requires four radiating elements 440, so the array only contains four radiating elements 440. Hence, the antenna cell used to support a specific transmit or receive function 10A,B,C actually shares the radiating element 440 and transmission line 430 with an antenna cell used to support another transmit or receive function 10A,B,C.
In an architecture where the radiating elements are shared or “reused,” passive couplers tend to introduce losses, so the use of diplexers or band pass filters is preferred. Tang et al. in U.S. Pat. No. 5,087,922 disclose bandpass filters coupled to dipole elements that present open circuits or short circuits at particular operating frequencies. Lee et. al in U.S. Pat. No. 4,689,627 disclose diplexers coupled to radiating elements in an array, where the diplexers provide isolation between the two frequency bands at which the array operates. However, reusing radiating elements in this manner may require the use of extremely complex and costly multiple band diplexers and/or wideband radiating elements.
Therefore, there exists a need in the art for an antenna array that can support multiple functions over extremely large bandwidths. There exists a further need in the art for an antenna array that provides improved isolation between signals at different operating frequencies, greater efficiency, and the flexibility to operate at several frequencies.